The present invention relates to a circuit for distinguishing a detected signal indicating the lifting of the valve element of a fuel injection valve which injects fuel into an internal combustion engine, and more particularly to a circuit for distinguishing a detected valve element lifting signal while removing noise from the detected valve element lifting signal to effect accurate detection of the fuel injection timing of a fuel injection valve.
The lifting of the valve element of a fuel injection valve is detected, for example, by an output signal which is generated by a pressure-sensitive means such as a piezoelectric element in response to the displacement of a member which is movable with the valve element of the fuel injection valve.
Since the pressure-sensitive means such as a piezoelectric element has a high output impedance, however, the output signal thereof is susceptible to noise, and may even pick up noise caused by the vibration of a valve nozzle spring by which the valve element is normally urged against a valve seat. Therefore, the valve element lifting signal is liable to oscillate due to such noise.
In order to distinguish a detected valve element lifting signal, it has been one conventional practice, as shown in FIG. 6 of the accompanying drawings, to count detected pulses (b) each produced upon detection of the top dead center, after a detected valve element lifting signal (a) has been issued, and to mask the signal for a period T up to a detected pulse (b) which is produced immediately before a next detected valve element lifting signal (a) is generated, thus removing noise from the detected valve element lifting signal.
According to another conventional scheme, as shown in FIG. 7, after a detected valve element lifting signal (a) has been produced, noise is masked or removed from the signal by a one-shot multivibrator.
With the former known signal distinguishing circuit, however, noise produced in a period (FIG. 6) after the masked interval cannot be removed.
The latter known circuit arrangement is disadvantageous in that noise in a period (FIG. 7) cannot be eliminated unless the masked interval according to the one-shot multivibrator is increased. However, if the masked interval is increased, it may also mask a next detected valve element lifting signal when the engine rotates at high speed.
The above two arrangements may be combined into a system in which the signal is unmasked at the time whichever masked interval ends first. Even with this system, however, noise cannot be removed from the period of time after the signal is unmasked until a next cycle of fuel injection is started.
There has also been a circuit arrangement in which the frequency of the vibration of the valve nozzle spring is removed by passing the output signal from the piezoelectric element through a low-pass filter. Where the low-pass filter is of an analog filter, it is difficult to provide a sharp decline in its frequency characteristic curve at the cutoff frequency. If the analog low-pass filter is successfully designed with a sharp cutoff decline in the frequency characteristic curve, then the low-pass filter has difficulty in detecting a positive-going edge of the output signal from the piezoelectric element, with the result being that a large detection delay will be produced.